FR2565844A1 - NOVEL MICROAGREGATES OF NON-NOBLE METALS, PROCESS FOR THEIR PREPARATION AND CATALYSIS APPLICATION OF PHOTOREDUCTION OF WATER - Google Patents

Abstract

THE INVENTION RELATES TO METAL MICROAGREGATES. THE MICROAGREGATES OF THE INVENTION ARE MICROAGREGATES OF NON-NOBLE METALS IN THE FORM OF A MONODISPERSE PHASE IN A LIQUID OR SOLID MEDIUM, AND OF SIZE UNIFORMLY LESS THAN 50A. THEY ARE OBTAINED BY IRRADIATION OF A METAL SALT, OR A MIXTURE OF SALTS, BY MEANS OF AN IONIZING RADIATION SOURCE, BY DISSOLVING A NON-NOBLE METAL SALT OR A MIXTURE OF METAL SALTS, AT LEAST ONE IS A NON-NOBLE METAL, IN AN APPROPRIATE SOLVENT, IN THE PRESENCE OF AN OXIDIZING RADICAL INTERCEPTOR, AND A SURFACING AGENT AND OR A CARRIER, THEN BY PERFORMING IRRADIATION. APPLICATION TO THE CATALYSIS OF VARIOUS REACTIONS, IN PARTICULAR THE PHOTOREDUCTION OF WATER IN HYDROGEN.

Description

The present invention relates to new metal micro-aggregates, and

  more particularly, new monodisperse phases of metal microaggregates of non-noble metals, a method of preparation by reduction of metal salts by radiolysis, as well as their application.

  cation as reaction catalysts.

  Various physical, chemical or physicochemical methods are known for preparing dispersed metal phases. For example, the metal may be mechanically milled and then dispersed in the medium chosen, or a condensation on a support, of a metal previously vaporized under vacuum. It is also possible to operate by ion reduction with molecular hydrogen at elevated temperature, or by basic etching of a metal alloy, for example to prepare Raney nickel. Physicochemical methods may include reducing metal ions on the surface of an electrode, discharging into a solution of metal ions or reducing

photochemical.

  However, these known methods do not make it possible to obtain the metal in the form of microaggregates of size less than a few nanometers dispersed uniformly in a medium.

both liquid and solid.

  It has also been proposed to use ionizing radiation

  sant (photons y, X or accelerated electrons) to reduce

  salts of noble metals with the help of solid electrons.

  vats formed in the solvent. This method has the advantage of producing solvated electrons in all points of the liquid, even inside the micropores of a cellular support used if necessary. However, the metals thus prepared belong to the group of noble metals (Ir, Pt, Pd, Rh, Ru, Au, Ag, Cd, T1) whereas the process does not make it possible to obtain in stable and highly divided form, other metals, such as nickel, which are however often used in various catalysts of many reactions. These difficulties result from the fact that the non-precious metals are characterized by a redox potential more conducive to corrosion, shifted further to values

  unfavorable for near-atomic aggregates. The pro-

  production of metals in a state of extreme division can not

  be successful in avoiding at least partially the reaction

  reverse corrosion during the nucleation phase of the aggregate. Beyond a certain size, particles

  are stabilized vis-à-vis the environment.

  Another difficulty appears in the case of multi-ion

  valents for which progressive radiolytic reduction involves partially reduced states. If this reduction process has the advantage of ensuring the dispersion of the incipient metal atoms and of controlling their aggregation, it presents, as any method based on the reduction of ions, risks of oxidation in return of the states

  intermediaries, risks which add to those of the corro-

  aggregates at the earliest stage of their growth.

  The present invention aims to prepare phases

  monodisperse microaggregates of non-noble metals, possessed

  having specific physicochemical characteristics,

  useful in particular as reaction catalysts.

  The subject of the invention is therefore monodisperse phases of microaggregates of non-noble metals, such as nickel, cobalt, copper, mercury or lead, or alloys of such metals with noble or non-noble metals, of dimension less than 5 nm, obtained without chemical reducers, by

  in situ reduction of metal salts by means of

ionizing events.

  The subject of the invention is also the application of micro-

  aggregates of non-noble metals as reaction catalysts,

  especially for the photoreduction of water in hydrogen.

3 2565844

  The invention further relates to a method for preparing

  radiolytically facilitate new microaggregates

  non-noble metals in the form of monodisperse phases

  forms, in a liquid or even solid medium.

  The process according to the invention consists in irradiating a metal salt, or a mixture of salts, by means of a source of ionizing radiation, and is distinguished by dissolving a non-noble metal salt or a mixture of salts. of metals

  at least one is a non-noble metal, in a suitable solvent

  in the presence of an oxidizing radical interceptor, and a surfactant and / or carrier, and then

irradiation.

  The oxidant radical interceptor used in the process of the invention is preferably a primary or secondary alcohol such as isopropanol, or a formate such as an alkali metal formate. It may be advantageous to choose the formate of the metal to be reduced, for example nickel formate. This compound is used to

  intercept the oxidative radicals formed in the solution.

  Indeed, the radiolysis of the solutions produces, in addition to the solvated electrons, radicals, such as the OH radical in the case of water, which can oxidize in return a

  part of the atoms resulting from the reduction by

  very solvated. The addition of a primary or secondary alcohol

  RHOH, which reacts rapidly with OH 'radicals to give new reducing R'OH radicals, makes it possible to increase the yield of metal. We can also

  use the formiate ion HCOO- as a radix interceptor

  cal. The product of the reaction is then the COO radical

  which also has an excellent reducing power.

  The concentration of the radical interceptor is determined according to the amount of metal to be produced,

  and therefore the necessary dose. The concentration of inter-

  radical receiver may be for example between

-2 and mol ..

_ and 1 mol.1l

4 2565844

  The metal yield for a given dose of ionizing radiation is higher for the higher concentrations of metal salts. However, if the concentration is too large, the greater ionic strength of the medium accentuates aggregations of colloids and particles.

  obtained then have a smaller size.

  The solvent used is chosen as a function of the metal salt, and may be, for example, water, acetonitrile, ammonia, tetrahydrofuran, ethylenediamine or an alcohol. The case

  If appropriate, a usual wetting agent may also be used.

  As indicated above, it is carried out in the presence of a surfactant and / or a carrier, that is to say in a liquid or solid medium. The surfactant preferably used

  is polyvinyl alcohol, in a concentration (in mono-

  mother) equal to several times, for example 10 times, that of metal atoms. Instead of the surfactant, it is possible to use a suitable support such as silicas, resins, oxides (Al 2 O 3, RuO 2e TiO 2), zeolites,

  carbon or insoluble salts such as stron-

  tium or cadmium sulphide, and metal microaggregates

  They are then deposited on this support.

  According to a form of implementation of the process of the in-

  vention, one can operate in a basic medium, and the pH can be for example increased to 9 or more, and preferably between and 11, by addition of sodium hydroxide or ammonia. However, the pH must not be too high in order to avoid precipitation of the metal hydroxide by hydrolysis with the solvent.

  On the contrary, a pH that is too low (acidic or weakly

  that) is unfavorable to the stability of metal microaggregates

  lic. Indeed, it is necessary to avoid any corrosion or reoxidation

  dation of the nascent metal under the action of oxidants present

  in the middle. Now the solvated proton H + can have a

  oxidant, and it is known that the formation of a phase

2565844

  metal divided from the corresponding salt causes a gradual acidification of the medium. Also metal aggregates do not form if the medium is initially

  acidic or weakly basic, at least to increase sensi-

  the concentration of the metal which may constitute a

  disadvantage for the control of nucleation.

  Operating under conditions of basic pH in the presence of a surfactant such as polyvinyl alcohol and

  an oxidative radical interceptor such as isopro-

  panol, we can significantly improve the production of

  microaggregates of metals such as rhodium and lead.

  It may be advantageous, in accordance with the invention, to facilitate the nucleation of metal atoms such as Ni,

  Co, Fe, Cu, Hg, producing them in the presence of microagre-

  gats of a noble metal such as palladium or platinum whose preparation is known elsewhere. These metals, for example platinum or palladium, may be added in small amounts, for example from 0.1 to 2 atomic% of the metal to be reduced. Larger amounts, greater than 2%, can be used to produce alloys containing platinum or palladium. They can be prepared by adding in advance a suspension of palladium or platinum previously reduced, or by carrying out the radiolytic reduction of a mixture of salts. We obtain

  thus, depending on the amount of noble metal added suspended

  a few% of nickel in Pt or Pd at

  core of the aggregate or distributed as in an alloy.

  Ni, Co, Pb, Hg, Cu, Cd alloys can be prepared in all proportions in Pd or Pt and FeCu alloys. Analyzes of diffraction patterns show that the atoms are regularly arranged in the network sites, as in a solid solution when the metals

are miscible.

6 2565844

  According to a variant of the process of the invention, in particular

  in the case of nickel, it is advantageous to perform the irrational

  diation in the presence of oxygen. Indeed, the addition of a

  based on the solution, associated with the presence of oxygen at

  reduced reduction, protects the incipient atoms from oxidation in return, by preventing, by complexation with oxygen, the inverse reaction of the intermediate valence with the medium. In the case of a metal salt solution of low concentration (some 3 mol.) The metal is not formed under irradiation in basic medium and in the presence of inert gas, or under vacuum. In the presence of air, the reduction does not occur either because of the reaction of

  reducing agents, solvated electrons, with oxygen.

  On the contrary, the addition of an oxygen pressure of

  About 50 torr allows the metal colloid to form.

  This oxygen supply must be removed after irradiation

  not to harm the stability of the colloid.

  Finally, we obtain a transparent brown solution

  of subcolloidal solutions. For some

  rate (Cu, Ni, Pb, Co, and alloys beyond a certain proportion of these metals) the color disappears in contact with the air, which reflects the reoxidation of the metal. Solutions must therefore be stored under vacuum or in an atmosphere

inert (nitrogen, argon).

  According to another variant of the process of the invention, it may be advantageous to increase the concentration considerably.

  surfactant. In this case the radiolytic reduction

  However, the pH may, if appropriate, be carried out at a relatively low pH, for example close to 7, without the need to add a base to the solution. For example, for a

  solution containing as surfactant polyalcohol

  vinyl concentration of 0.2 mol.l-, the concentration in

  Ni2 + being 2x10-3 mol.1-, the reduction of the metal is

  under vacuum in initially-neutral medium.

C 2565844

  The concentration of metal salt can be between 4 and 10-1 mol. ll, and preferably between 10- 3 and 10-2 mol.l1l approximately. It may be advantageous, particularly in the case of the niokel, to use a salt with a concentration of between 10 -2 and 10 -1 mol / l to facilitate the aggregation of the metal. The microaggregates of nickel can then be formed in the presence of a radical interceptor and a surfactant or a porous support without base addition,

  of oxygen, platinum or palladium. For example, the solution

  tion may contain the nickel salt (5xl0-2 me.l-1), the

  cool polyvinyl (some 10-1 mol.i-I monomer) and isopropanol concentration in relation to the amount

  desired nickel (for example 10 -2 to 10 mol -1). The ir-

  radiation forms nickel atoms at the rate of one atom per

  200eV of energy absorbed approximately (50eV in the case of

  miate). The metal salt may be chosen from the usual salts of the metals that it is desired to reduce. We can use for example

  sulphate, chloride, perchlorate, metal formate

  lic. Formate has the advantage of providing an anion

  who plays the role of radical interceptor.

  Irradiation is performed using any source

  ionizing radiation, at doses that are

  tion of the amount of metal and which can be for example of the order of 50 to 700eV per metal atom. For example, a y (cobalt 60) radiation source of 0.2 to 0.5 M.rad / h can be used for 1 to 20 hours. The ferromagnetic particles (Fe, Co, Ni) can optionally be

  separated from the solution by a magnetic field.

  As indicated above, the process of the invention makes it possible to prepare monodisperse phases of metal microaggregates.

  non-noble, associated where appropriate with a cometal,

  the size of microaggregates which is lower than

8 2565844

  less than 5 nm, and preferably less than about 3 nm, and by their uniform dispersion in the solution or on the support, even inside the micropores of a cellular support. In addition, microaggregates of non-noble metals or alloys, in stable form, which can be used in

various applications.

  The non-noble metal may be chosen from nickel,

  cobalt, lead, mercury, iron, copper, rhodium.

  The cometal used in combination can be chosen from

  same metals or noble metals, and preferably

tine and palladium.

  The microaggregates obtained can be used as cataract

  reduction lysers for all catalyzed reactions

  usually by Raney nickel. They can be used

  alone or on a support, in particular

  for the photoreduction of water into hydrogen.

  It is known that the effectiveness of a heterogeneous catalyst depends on

  essentially the speed of the reversal inter-reactions

  with the molecules at the interface between phases, and therefore the degree of dispersion of the catalyst. It is therefore particularly useful to be able to have catalysts, especially finely divided metal catalysts (especially in the form of microaggregates) in stable and homogeneous form. With regard to photosensitized dissociation of

  water for the purpose of conversion and storage of

  it is known that the use of a catalyst is essential

  sand for the concerted transfer of several electri-

  trons. It is found that the activity of nickel microaggregates in accordance with the present invention is comparable to that of the best known catalysts of this reaction. Thus, for the same concentration, the nickel microaggregates have a

9 2565844

  activity equal to about 80% of that of platinum, while the activity of Raney nickel is practically nil. The

  microaggregates according to the invention thus prove to be particularly

  economically advantageous.

  The examples given below illustrate the invention more

detail without limiting its scope.

  In the case of the preparation of a catalyst for the dry route, the support is impregnated to the pore volume with the solution of metal ions. In the case of a wet catalyst, the ionic solution is either the divided carrier or the surfactant (polyvinyl alcohol).

EXAMPLE 1

  A subcolloid nickel solution is prepared at the concentration

  2xlO3 mol.1-1 as indicated below.

  A commercial solution of nickel sulphate is diluted to the title chosen (2 × 10 3 mol.l -1, ie 0.11 g / l). Polyvinyl alcohol is added at a rate of more than 10 times more than 2-ol monomers than metal atoms (4.5 × 10 -2 mol -1).

2 g.1i1).

  To this solution was added isopropyl alcohol at a concentration of 0.28 mol.l 1, then palladium chloride (concentration 2xlO 4 mol.l-1). A few drops of 0.5M sodium hydroxide are then added to bring the pH to about 10.6, then the solution is pumped off, and irradiated with a source of γ (cobalt-60) radiation at the rate of 700eV.

  metal atom (ie 3.3x1020 eV.g 1). The duration of the irra-

diation is about 15 hours.

  This gives a transparent brown solution characterized

  Subcolloidal solutions remain stable and do not sediment, even after several months of storage under an inert atmosphere. Electron microscopic examination

  shows that the size of the microaggregates is less than 3 nm.

1o 2565844

  The solvent can be removed by evaporation for prepa-

  a catalyst for the dry route.

EXAMPLES 2 - 16

  By proceeding as in Example 1, in the presence or absence of palladium chloride, or by replacing it with copper sulphate (eg 8 and 9), if necessary without the addition of sodium hydroxide, microaggregates are prepared. metal in the

  conditions shown in the table below.

  Ex. Metal salt. pH Cometal Alcohol Formiate P.V.A. Dose N Conc.Mol.1- Conc. Mol.1-Mol.1-Mol.-1 Mol.1-1020eV.g-1 NiSO 2x10-3 10.6 PdCl2 2x10 -4 0.28 4.5x10.2 3.3 2 RhCl3O3 8.9O, 56 10-2 0.72 3 NiSO4 3x10 -3 10.2 PdCl2 10-5 0.8 0.14 3.3

4 NiSO4 2xO-

  4 NiSO4 2x103 8 0.56 (*) 3.3 CoSO4 10-3 9.2 PdC12 5x10 -5 0.56 9x10-2 1.32

4 2

  6 HgC12 10-3 he PdC12 10-4 0.56 9xlO-2 0.35 7 CuSO4 10-3 10.2 PdCl2 104 0, 56 0.1 0.33

8 FeSO2x10 -3 -

  8 FeSO4 2x10-3 10.4 CuSO4 2x10 -3 0.11 0.2 3.3 9 NiSO 5x104 6.7 CuSO4 5x104 0.056 0.2 1.1

4 4

  CuSO4 2x10-3 10.4 0.56 0.2 3.3 il PbClO4 2x10-3 8 0.11 2x10-2 0.4 12 NiSO4 2x10 -8 8.8 (**) 0.56 9x10-2 3 , 3 13 NiS04 2x10 -3 6,7 0,28 0,2 3 14 NiSO4 5x10 -6,7 0,028 0,2 1,1 r) NiSO4 5x102 6,7 0,1 0,55 3,3 in 16 NiSO4 1,5x102 6,7 0,56 (*) 1,76 (*) SiO2 (spherosil) (**) pressure 02: 50 torrs

12 2565844

Claims (17)

R E V E N D I C A T I 0 N S   1. New microaggregates of non-noble metals characterized in that they are in the form of a monodisperse phase in a liquid or solid medium, and of uniformly smaller size at 5nm.   2. Microaggregates of non-noble metals according to the   1, characterized in that the size of the microaggregates is less than 3nm.   3. Microaggregates of non-noble metals according to some   of claims 1 and 2, characterized in that the   Non-noble metal is associated with a cometal.   4. Microaggregates of non-noble metals according to the claim   3, characterized in that the cometal is selected from   palladium, platinum and copper.   5. Microaggregates of non-noble metals according to any   than the preceding claims, characterized in that   the non-noble metal is chosen from nickel, cobalt,   lead, mercury, iron, copper and rhodium.   6. Microaggregates of non-noble metals according to some   of claims 1 to 5, characterized in that   are in the form of a subcolloidal solution.   7. Microaggregates of non-noble metals according to any   claims 1 to 5, characterized in that they are   in the form of a deposit on a support chosen from silicas, resins, oxides, zeolites, carbon and insoluble salts such as strontium titanate or sulphide of cadmium.   8. Process for the preparation of microaggregates of non-noble metals in homogeneous dispersed phases by irradiation of a metal salt, or a mixture of salts, by means of a 13 2565844   source of ionizing radiation, characterized in that a non-noble metal salt or a mixture of metal salts, at least one of which is a non-noble metal, is dissolved in a suitable solvent in the presence of a radical interceptor oxidants, and a surfactant and / or a carrier, and irradiation is carried out.   9. The method of claim 8, characterized in that the oxidizing radical interceptor is a primary or secondary alcohol such as isopropanol. or a formate such than an alkali metal formate.   10. Process according to claim 9, characterized in that   the radical interceptor is nickel formate.   11. Process according to any one of claims 8 to   characterized in that the surfactant is polyvinyl alcohol in a concentration (monomer) equal to   several times that of metallic atoms.   12. Process according to any one of claims 8 to   characterized in that the carrier is selected from silicas, resins, oxides, carbon, zeolites, and insoluble salts such as strontium titanate or cadmium sulphide.   Method according to claim 8, characterized in that the pH is between 10 and 11.   14. Process according to claim 8, characterized in that the irradiation is carried out in the presence of oxygen at reduced pressure.   15. Process according to any one of claims 8 to   14, characterized in that the concentration of the metal salt is between -4 and 1 mol.1.   is between 10- and 10- rnol.1 14 2565844   16. Process according to any one of claims 8 and   13, characterized in that palladium or   in the form of metal previously divided or in the form of   of salt, in a proportion of 0.1 to 2 atomic% of the metal to be reduced.   17. Catalysts of photoreduction of water in hydrogen   characterized in that they are constituted by micro-organisms   metal gats according to any one of the claims 1 to 7.